This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-241132, filed Aug. 8, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor memory device and, more particularly, to a magnetic random access memory (MRAM) using a tunneling magneto resistance element (TMR) as a memory element.
2. Description of the Related Art
Recently, an MRAM (Magnetic Random Access Memory) cell using a magneto resistive effect is proposed as an information memory element. This MRAM has developed into a memory device having nonvolatile properties, high integration degree, high reliability, and high operating speed, and hence is increasingly expected in recent years as a device having potentials.
A GMR (Giant Magneto Resistive) element and a TMR (Tunneling Magneto Resistive) element are primarily known as this magneto resistance element. A GMR element has two ferromagnetic layers and a conductor sandwiched between these ferromagnetic layers. The resistance of this conductor changes in accordance with the magnetization directions in the upper and lower ferromagnetic layers. However, since the MR (Magneto Resistive) ratio of this GMR element is as low as 10%, a read margin is difficult to ensure. Therefore, GMR elements are limited to special purposes and have not been widely used yet. A TMR element has two ferromagnetic layers and an insulator sandwiched between these ferromagnetic layers. The tunnel resistance of this insulator changes in accordance with the magnetization directions in the upper and lower ferromagnetic layers. This TMR element can presently assure an MR ratio of about 50%. Hence, TMR elements have been studied as application devices more frequently than GMR elements.
Accordingly, an MRAM memory cell uses TMR elements as a memory element and a reference element. In data read, the resistance value of the memory cell is compared with that of the reference element to check whether the data is xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d.
Unfortunately, the above prior art has the problem that variations in the resistance value of the reference element reduce the resistance difference between data xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d. It is, therefore, necessary to suppress variations in the resistance value of the reference element.
A magnetic memory device according to an aspect of the present invention comprises a first resistance element formed in a memory cell unit, and at least one second resistance element and at least one third resistance element formed in a reference cell unit, the first, second, and third resistance elements storing binary data by a resistance change, the second resistance element storing one of the binary data, and the third resistance element storing the other of the binary data.